This invention relates generally to ion and plasma technology, and more particularly it pertains to ion-assisted deposition from magnetrons wherein the ions from an ion source compact, react with, or otherwise modify the thin film deposited by a magnetron.
Deposition of thin films by sputtering is widely used. The technology of magnetrons with planar targets is described by Waits in Chapter II-4 of Thin Film Processes (John L. Vossen and Werner Kern, eds.), Academic Press, New York (1978) beginning on page 131. Sputtering targets with other than planar shapes have also been used in magnetrons, as described in U.S. Pat. No. 2,146,025xe2x80x94Penning and in U.S. Pat. No. 3,616,450xe2x80x94Clarke. A related deposition technology is the use of energetic ion beams directed at sputter targets to deposit thin films on substrates as described by Harper in Chapter II-5 of the aforesaid Thin Film Processes beginning on page 131. These publications are incorporated herein by reference.
The modification of thin films by the simultaneous bombardment of a depositing film with energetic ions is called ion-assisted deposition. The acceleration of ions to form energetic beams of such ions has been accomplished electrostatically as described in U.S. Pat. No. 3,156,090xe2x80x94Kaufman and in the aforementioned chapter II-4 by Harper in Thin Film Processes. The ion generation in these ion sources has been by a direct-current discharge. It is also possible to use electrostatic ion acceleration with a radio-frequency discharge, as described in U.S. Pat. No. 5,274,306xe2x80x94Kaufman, et al.
The acceleration has also been accomplished electromagnetically with a discharge between an electron-emitting cathode and an anode. The accelerating electric field is established by the interaction of the electron current in this discharge with a magnetic field located in part or all of the discharge region. This interaction generally includes the generation of a Hall current normal to both the magnetic field direction and the direction of the electric field that is established. For the Hall current to be utilized efficiently, it must traverse a closed path.
A Hall-current ion source can have a circular discharge region with only an outside boundary, where the ions are accelerated continuously over the circular cross section of this region. This type of Hall-current ion source is called an end-Hall ion source and has a generally axial magnetic field shape as shown in U.S. Pat. No. 4,862,032xe2x80x94Kaufman et al., and as described by Kaufman, et al., in Journal of Vacuum Science and Technology A, Vol. 5, No. 4, beginning on page 2081. These publications are incorporated herein by reference.
A Hall-current ion source can also have an annular acceleration region with both inner and outer boundaries, where the ions are accelerated only over an annular cross section. This type of Hall-current ion source is called a closed-drift ion source and usually has a generally radial magnetic field shape as shown in U.S. Pat. No. 5,359,258xe2x80x94Arkhipov, et al., and U.S. Pat. No. 5,763,989xe2x80x94Kaufman, and as described by Zhurin, et al., in Plasma Sources Science and Technology, Vol. 8, beginning on page R1.
As described in the prior-art discussion of the aforesaid U.S. Pat. No. 5,763,989, closed-drift ion sources can be divided into magnetic layer and anode layer types, with the presence of a dielectric wall and a longer discharge region being the primary distinguishing features of the magnetic layer type. The operation of the anode-layer type can be further divided into quasineutral and vacuum regimes. As further described by Zhurin, et al., in the aforesaid article in Plasma Sources Science and Technology, operation in the vacuum regime is characterized by a high discharge voltage and a low discharge current. All of the above described Hall-current ion source types and their operating regimes use an electron-emitting cathode such as a hot filament or a hollow cathode, except for the anode-layer type operating in the vacuum regime. For the electron current to sustain the discharge, the latter depends on electron emission from cathode-potential surfaces that results from ion bombardment of those surfaces, from field-enhanced emission, and from neutralization arcs. An example of an anode-layer type of ion source operating in the vacuum regime is described in U.S. Pat. No. 6,147,354xe2x80x94Maishev, et al. The above ion source publications are also incorporated herein by reference.
The cross sections of the acceleration regions in the preceding discussion are described above as being circular or annular, but it should be noted the cross sections can have other shapes such as an elongated or xe2x80x9crace-trackxe2x80x9d shape. Such alternative shapes are described in the references cited. It should also be noted that the magnetic field shape can depend on the desired beam shape. For example, an ion beam directed radially outward would have a magnetic field generally at right angles to the magnetic field used to generate an axially directed ion beam.
Ion-assisted deposition has been carried out using ion-beam sources for both sputtering from a target and the ion-assist bombardment of the depositing film. In such deposition, a gridded ion source is almost always used for generating the ion beam directed at the sputter target, as described in U.S. Pat. No. 4,419,203xe2x80x94Harper, et al., and U.S. Pat. No. 4,490,229xe2x80x94Mirtich, et al.
Ion-assisted deposition has also been carried out using a magnetron to sputter from a sputtering target and an ion source for the generation of ion-assist ions, as described in U.S. Pat. No. 5,525,199xe2x80x94Scobey and U.S. Pat. No. 6,153,067xe2x80x94Maishev, et al.
Ion-assisted deposition can enhance properties of deposited thin films by increasing their density, increasing their hardness, modifying their stress, promoting crystalline alignment, selecting a preferred molecular bond, increasing their adhesion to the substrates upon which they are deposited, and promoting the formation of a particular compound (such as an oxide or nitride) by bombarding with ions of one of the elements (such as oxygen or nitrogen ions). These property enhancements are described by Harper, et al. in Chapter 4 of Ion Bombardment Modification of Surfaces: Fundamentals and Applications (Auciello, et al, eds.), Elsevier Science Publishers B. V., Amsterdam (1984), beginning on page 127; by Kay, et al. in Chapter 10 of Handbook of Ion Beam Processing Technology (Cuomo, et al., eds.) Noyes Publications, Park Ridge, N.J. (1989) beginning on page 170; and by Roy, et al. in Chapter 11 of the aforesaid Handbook of Ion Beam Processing Technology, beginning on page 194. The above ion-assisted deposition publications are also incorporated herein by reference.
An acceptable energy range for ion-assist ions in low-damage deposition can be determined from the above publications. Essentially all deposition processes appear to show some degree of damage at ion-assist energies above 300 eV. Quite a few processes show damage at energies greater than 100 eV, while some show damage at energies greater than 50 eV. For low-damage, ion-assisted applications, then, the ion energies should definitely be less than 300 eV and preferably less than 100 eV. In general, gridded ion sources as described by Harper in the aforesaid Chapter II-5 of Thin Film Processes have limited ion-beam current capacity at low energies and are therefore not well suited to ion-assist deposition at the low end of the  less than 300 eV energy range. In comparison, Hall-current ion sources usually have substantial ion-current capacity at 100 eV, or even less.
A general trend in thin-film deposition has been the increasing suppression or elimination of damage-producing mechanisms, with the objective of producing deposited films that are nearly or completely free from damage. The control of impurities has been important in reducing damage in the form of departures from uniform and controlled composition.
Impurities have been decreased by reducing or eliminating the sputtering of non-target hardware. A general reduction of contamination has also been obtained by reducing the vacuum-chamber pressures. While low operating pressures can be important, the use of vacuum chambers with low pressures prior to operation is probably more important. This is because high-purity process gases can be used to reduce impurities during operation, but a high pressure prior to operation means that impurities from outgassing and/or leaks will be present during operation regardless of the purity of the working gases. Load-lock vacuum systems, where the vacuum chamber remains at vacuum while substrates are introduced and removed from the vacuum chamber through auxiliary vacuum chambers are effective for reducing the contamination due to outgassing. To offset the reduced impurities, however, the sputter equipment used in such systems must be capable of operating for long times between maintenance.
A more recent contribution to the deposition of low-damage films has been the control of substrate charging. This control is obtained by controlling the plasma potential in the vacuum chamberxe2x80x94i.e., by keeping it close to the potential of the substrate and the surrounding vacuum chamber. Large excursions in plasma potential can cause the neutralization arcs mentioned in connection with an anode-layer ion source operating in the vacuum regime. These arcs can also be generated when other types of ion sources are operated at improper conditions. These are arcs between the plasma and the substrate or between the plasma and the surrounding hardware. Neutralization arcs have a very short duration and a typical visible length of 1-2 mm. When such arcs are frequent, they can give a sparkling appearance in a dimly lit vacuum chamber. Damage from neutralization arcs can be in the form of arc pits at the origin of the arcs or particulates from these arcs being deposited elsewhere.
Reduction or elimination of neutralization arcs in a gridded ion source has been obtained by operating the cathode-neutralizer emission at a value equal to or slightly greater than the ion beam current, as described by Kaufman, et al. in Operation of Broad-Beam Ion Sources, Commonwealth Scientific Corporation, Alexandria, Va. (1987), starting with page 49. For a Hall-current ion source, the reduction or elimination of neutralizing arcs has been obtained by operating the cathode at an emission value equal to or slightly greater than the discharge (anode) current as also described by Kaufman, et al., in the aforesaid Operation of Broad-Beam Ion Sources, starting with page 60.
The most recent work on reducing substrate damage by reducing substrate charging has focused not just on the elimination of neutralization arcs, but on the reduction of substrate charging to well below the level where such arcs are observed. Problems have been encountered with electrostatic charging during ion beam etching, as described in an article by Olson in the EOS/ESD Symposium, Paper No. 98-332 (1998), also incorporated herein by reference. These problems have been most serious when portions of the work piece at which the ion beam is directed are electrically isolated from each other. Differential charging of these isolated portions can result in an electrical breakdown between the two portions. Such a breakdown will damage the work piece.
In light of the foregoing, it is an object of the invention to provide an apparatus using a magnetron sputtering source that can be used in ion-assisted deposition to deposit films that are substantially free of damage sites.
Another object of the invention is to provide an ion-assisted magnetron deposition apparatus that does not require operation of an electron-emitting cathode.
Yet another object of the present invention is to provide an ion-assisted magnetron deposition apparatus that is capable of extended operation without maintenance.
Still another object of the present invention is to provide an ion-assisted magnetron deposition apparatus in its simplest, most reliable form.
In accordance with an embodiment of the present invention, apparatus for ion-assisted magnetron deposition takes a form that includes a magnetron, a deposition substrate displaced from the magnetron, and an ion source also displaced from the magnetron and located so that the ion beam from the ion source is directed toward the deposition substrate. The ion source is operated without an electron-emitting cathode-neutralizer, the electron current for this function being provided by electrons from the magnetron. In one specific embodiment, the ion source is operated so that the potential of the deposition substrate is maintained close to that of a common ground for the magnetron and the ion source. In another embodiment, the ion source is of the Hall-current type and the discharge current of the ion source is approximately equal in magnitude to the current of the magnetron discharge.